During operation, telescopic crane jibs are exposed to loads which result primarily in tensile stress in the upper part of the jib cross section, i.e. roughly in the upper half of the cross-section of the telescopic part. Horizontal bending and torsion can also occur due to lateral forces (wind) and off-center loads.
The cross-sectional shape of the upper part of earlier jib sections might be characterized as semi-box shaped profiles or cross-sections as described, for example, in DE 196 24 312 A1. Upper cross-sections for jibs which are adapted in shape were then later described, for example in DE 200 04 016 U1 and in EP 1 321 425 A1. The latter upper portion cross-sections comprised a central flat cross-sectional element and other flat and outwardly curved cross-sectional elements.
It is the object of the present invention to provide a cross-sectional configuration for the upper part of a telescopic crane jib which offers an optimised measure of bearing capacity as well as simplicity of manufacture.
In accordance with the invention, a cross-section for the upper part of a telescopic jib for a crane includes a central flat cross-sectional element. On each side of the central flat element there is connected in succession a first outwardly curved cross-sectional element; a second flat cross-sectional element; a second outwardly curved cross-sectional element; and a third flat cross-sectional element.
The costs of shaping telescopic parts form a substantial portion of the overall manufacturing costs for a crane, and manufacturing costs should be kept as low as possible. On the other hand, the cross-section of a jib should be able to absorb the imposed loads as well as possible. Both of these objectives are achieved with the configuration in accordance with the invention. The central flat cross-sectional element extends on both sides of the vertical longitudinal plane of the telescopic part of the crane, and the aforementioned additional cross-sectional elements are each provided on both sides of this plane. Such a cross-sectional design optimises the stability of the jib while providing also for ease in manufacturing. Using the outwardly curved cross-sectional elements and the flat cross-sectional elements in accordance with the invention creates a number of deflections within the upper part of the jib cross section which act as idealised stiffeners to counteract buckling. For luffing jib operations, however, this is also highly advantageous in pre-tensioned and/or braced jib systems, and the necessity for providing separate stiffeners to counteract buckling is minimised or completely eliminated.
In particular, providing cross-sectional elements in the numbers, shape and arrangement in accordance with the invention has the effect of providing deflections in the lateral cross-sectional parts, such that the individual lateral areas prone to buckling are more sharply delineated and the overall buckling field is reinforced unlike, for example, the relatively large and/or long individual buckling areas provided in accordance with DE 200 04 016 U1. The present invention thus increases the resistance to lateral buckling.
The outwardly curved cross-sectional elements in accordance with the invention can be configured using a single tool and in one canting process, resulting in a total of four deflections or curvatures in the upper chord (upper shell) as a whole. This leads to easier manufacturability and lower costs as compared, for example, to generation of curved elements which are expanded and connected to each other as in EP 1 321 425 A1. The flat (or planar- or linear-running) cross-sectional elements afford the option of positioning the canting tool very precisely and, thus, ensure high process reliability. The present invention, thus, achieves an optimum synthesis of manufacturing optimisation and stability optimisation.
In accordance with one embodiment of the invention, the third flat cross-sectional element noted above runs parallel to the vertical longitudinal plane of the telescopic part of the crane and forms the lowermost or termination of the upper cross section. Due to such an arrangement, the lower end of the upper cross-section runs linearly or vertically downwards and can, therefore, easily transition into and connect to a part of the lower cross section. This also contributes to achieving an optimised ability of the jib section to absorb force at the connecting point.
Preferably, the above-described upper cross-section forms substantially the entire upper half of the telescopic part, i.e. the lower termination of the upper cross sectional part is situated substantially level with the vertical middle of the jib cross-section. This places the connecting point (welding line) substantially in the zone which remains tension-free when a load is affixed, between the tensile stress zone and the compressive stress zone (top/bottom).
Advantageously, at least one and, preferably, all of the transitions between the flat cross-sectional elements and the outwardly curved cross-sectional elements run tangentially. This avoids stress peaks at the transitions.
With respect to their length and curvature, the cross-sectional elements in accordance with the invention can satisfy one or more of the following conditions:                the first outwardly curved cross-sectional element may be longer than the second outwardly curved cross-sectional element;        the central flat cross-sectional element may be longer than the second flat cross-sectional element (as discussed herein, the “central” flat cross-sectional element can also be regarded as the “first” flat cross-sectional element);        the second flat cross-sectional element may be longer than the third flat cross-sectional element;        the first outwardly curved cross-sectional element may be outwardly curved more sharply than the second outwardly curved cross-sectional element.        
Depending on the specifically desired characteristics, the length ratios and curvature ratios of the respective elements can be inverted, or identical lengths and curvatures can be provided for the respective elements. For smaller jib parts, for example, the second flat cross-sectional element might not be longer than the third flat cross-sectional element. The cross-sectional elements can be arranged, proceeding successively away from the central upper element, in precisely the order initially given above. It is also advantageous in accordance with the invention if the cross-sectional elements are arranged such that flat and curved elements alternate.
“Curvature” or “bend” as used herein mean gradual curved or arched transitions, as opposed to kinked cants or angled transitions (with and without welding seams).